Microwave plasma density measurement system



MICROWAVE PLASMA DEN sYs E provided.

3,265,967 f T I`sIIT-Y.Mi-:ASUREMENT I .Mark A. He'ald, Swarthmore,Psi., 'assignor to the United States of America as rcpresented by theUnited States Atomic Energy Commission continuation of application.'ser. No; 153,271, No'v. v16,

1961. This applicationlune 8, 1965, Ser. No. 462,457

3 Claims. (Cl. S24-58.5)

This application is a continuationl of application Serial No. 153,271filed November 16, 1961, now abandoned,v

entitledv Microwave Plasma Density Measurement System' y This inventionvrelates generally to a' microwave measuring system and moreparticularly toan' improved micro.

wave measuring system for determining the density of a plasma of lionsand electrons. i In the electronic and atomic energy tields, it has beennecessary. to produce plasmas of ions and electrons and to measure vtheplasma characteristicsincluding the plasma density. As described inControlled Thermonuclear Rennaedsas 1 Patent oice actions by Glasstoneand' Lovberg',` microwave systems have been advantageously .employed formeasuring plasma density due. to the fact that the probingmicrowaveshave been of such low power that they have caused practical-lyno disturbanceof the conditionsjn the' plasma. However,

has beendiflicult or impossible .to provide `a microwave system' forproducing continuousl foutpu't. .signaldirectly proportional totheplasmadensity below its critical density where-the plasma becomes opaqueto microwaves.

lthas been discovere-d in accordance with this invention, that theseproblems areovercorne and that -a continuous output signal can beprovided that vanies directly with the plasma density below the criticalplasma density, w-hile providinga stable reference signal when. theplasma highly attenuatesf microwaves passing therethrough,'by animprovedphase detection system. More particularly, this invention 'in oneembodiment contemplates having a frequency modulated signal sourcehavinga highly frequency sensitive microwave bridge with crystal detectors forgenerating avcarrierA frequency which is phase modulated by the-timevarying plasma sample vin onearm of the bridge,

a frequency modulation receiver comprisinga differential amplifier, alimiter-,fa discriminator and a video amplifier connected in a chain tothe crystal detectors 'of the bridge, .and an electronic integratingcircuit' .that is. operable-with vthe'chainseriati'rri to produce avoltage that varies linearly with the phase modulation o f V the saidsignal produced .by said detectors. With the proper selection ofcomponents and frequencies Ias described in moredetail hereinafter, thedesired output andpl-asmadensity determinations arev v'The above Aandfurther novel features of the invention will appear more fully-from thefollowing detailed descriponly. i

In the drawings'where like parts are marked alike:`

. FIG.'1 is a block diagramof a preferred circuit in accordance' withthe present invention for, accomplishing plasma density measurements.

.Referring now to FIG..l,`rneasur ing circuit 1-1 measures thedensity'of a plasma,le.'g., 'the free elec-tron concentral tion of aplasma in containerl, accordance with this' invention. The' plasmacontainer 13 may b'e a plasma gen- .'erator .such as -a Philips IonGage, a gas discharge tube,

a thermonuelearreactor tube sueltas is described in U.S.

. Patent No. 2,910,414 orContr'o lled'Thermonuclear Reactions byGlasstone and Lovberg whereA plasma densities may be below or above 1013electrons/cm?, or any other plasma container such as a ballistic tunnelfor the study The plasma of plasmacaused by high velocity projectiles.in chamber 13 is indicated generally at 15.

Measurement circuit l11 preferably includes a irst section 1'7 having afrequency modulated signal source 19, a highly frequency-sensitivemicrowave bridge 21, in one arm 23 of which is located the time varyingdielectric plasmasample 15, and microwave detecting means 25. Frequencymodulated signal source 19 includes a suitable power -source 27connected to a conventional klystron or backward-wave oscillator 29capable of producingmicrowaves at a frequency j, of about 35,000megacycles per second. A vmodulation oscillator 31 -produces a simplesine-wave intelligence signal of second frequency f2 of about 30megacycles per second which frequency modulates the rstor carrier waves.A radio-frequency transmission line- 33, commonly referred to as `amicrowave waveguide, has an appropriate size to deliverv theflmicrowaves through -isolator 35 to directional coupler 37 and plasma15 in container 13 produces with path -39 an interference signal atjunction 47 where the two paths 39 and 41 reunite.

-Waveguide 49 .makes transmission path 39 veryv much longer thanreference path 41 and this transmission path 39 contains suitabledirective antennas or horns 51 and 53 between which container 13 holdsthe time varying dielectric plasma sample whose density is to bemeasured.

In accordance wi-th the particular systemof this invention, -it has beenfound .that the plasma phase-modulates the above-described particularmicrowaves'passing through transmission or test path 39 and this phasemodulation corresponds to the' plasma density. This phase modulation isdetected by a balanced mixer comprisingA crystal detectors 215 whichproduce a signalv at the particular frequency f2 carrying the phasemodulation. This signal is applied to a frequency modulation vreceiver 61 and an integrating circuit 63` through a coaxial cable 65 and thisproduces a voltage which varies linearly with the phase-shift producedyby .the plasma sample 15. This voltage is fed to i an oscilloscope 67or tape-recorder or other data recording instruments soas to indicatethe plasma density directly without complicated calculations and to thisend residual amplitudernodulation of the microwave oscillator stabilizesvthe output signal-even when the microwaves are highly attenuated bythe-plasma or thc plasma density 'is high.

The frequency modulation receiver 61 or phasedetector 61 of thisinvention has a differential pre-amplifier 69 that amplifes higher andlower frequencies than the frequency f3 within a frequency b and f3determine-d by the time rate of change of plasma density with thefrequency f2 acting as a center frequency. Coaxial cable 65 transmitsthe fz ysignal to limiter 71 that removes any amplitude modulation fromthe f2 signal, i.e., retains only phase-modulation of Discriminator 73derives variations in the form of a video-frequency .voltage -inaccordance with the frequency modulation of the center frequency f2.Thereupon, ampliiier 75 boosts this video-frequency voltage andtransmits it to an electronic time-integrator 77 which recovers thevphase modulation envelope produced by the plasma and produces a voltagethat is proportional to the Batertted Aug-ust 9, 1966 Spenge?callyitime-integrated by intcgrator77 to recover the phasemodulationenvelope. v

In review of the above Klystron 29 produces microwaves at frequency f1and oscillator 31 produces a sine An example of the parameters of thesystem of this inventionA are: f1'=35,00(l megacycles'per second, f2=30megacycles per second, f3= 2 megacycles per second, and T :l second.providef1 f2 f3 1/T, Alsov the degree of frequency modulation, M='tf/j1, of oscillator 29 andthe lengthL of wa-veguide 49 must vberelated by the equation v M1 approximateiy equals W'whereW is thewavelength corresponding to the frequency f1.

In one embodiment, the crystal detector is a'M-icrowave lAssociatesdetector model w26-AR, the limiter isl a 30 rnc. LF. amplifierLEL modelLF. 20B, the discriminator is a 30 mc. VELS model 99, the videoamplifier is a Kicthley mode-H0213 and the integrator isan ELS model181.4

In operationklystron or backwardwave oscillator 29 of frequencyfhpowered by av suitable power supply27,

. is frequency modulated by an oscillator 3.1 of frequency f2. Themicrowave output power of oscillator 29 is delivered to aradio-frequency transmissionline such as waveguide 33. rWaveguide 33 ifof -a size appropriate to ithe `frequency f1 and is used to interconnectall elements I where N represents the number of interference cycles Inthis example, these frequencies must of thecircuit operating at `thefrequency f1, An isolat'or oi padding attenuator 35 preventsanyreflected microwave power from altering the frequencyor amplitude of theoutput of oscillator 29. The microwave power is divided by a hybridjunction or directional couplerffinto two waveguide transmission paths39 and 41. The power traversing these' two paths is later reunited atthe hybrid junction 47. One of these two 'paths is made very muchvlonger than the other vby the inclusion of additional waveguide 49;-the additional waveguide may be located in eitherpath 39 or` in path 41,whichever is more convenient.- IPath 39 contains a time-varyingdielectric sample 15, the properties of which it is the function'of thiscircuit to measure. Specifically, the sample may be atimevaryingionizedgas or plasma. The microwave power is 'beamed throughthe samplebymeans of suitable directive antennas 5 1 and 53. Theamplitude of microwave' power reaching junction 1 7 by path 41 isadjustedby means'of levelsetting attenuator 43, The -phase ofthefmicrowave signal reaching junction 47 by path 41 is controlled bya'calibrated,.variable phase shifter45,` for purposes 'of .calibratingthe output signal from the circuit. The output branches of hybridjunction `47 deliver'the microwave -power to microwave detector crystals25, which apply an electrical signal in thevic'inity of themfodulation'ffre- A.quency f2 to the differential pre-amplifier 69.Thecombin-ation of hybrid junction and detector crystals 25 and 251- issometimes known asa balanced mixer. Tirne variations of the relativephase of the'microwave signals reaching junction 47 by paths 39 and 41,on account of the action of sam-ple 1S or calibrationphase shifter 45,produce a'phasc-modulation of the signal at frequency .f2 applied to thepre-amplifier 69. To preserve this phasemodulation vthe pre-amplifiermust amplify frequencies in the band widthja on both sides of the centeifrequency f2. The transmission of signals near f2' between componentsoperating at that frequency is 'accomplished using exible coaxial cable65; The output'of pre-amplifier 69 is delivered to the amplifying andlimiting circuit 71, which removes any amplitude modulation. The outputofthe limiter 71 -is delivered to a discriminator 73, which Vprovides avideo-frequency output in Iaccordance with the frequency-modulation ofthe signal with center-frequency f2. f signal is amplied-byamplifier 7 5and electronidispersion, such that wave signal at frequency f2 thatmodulate the microwaves. Coupler 37 splits the modulated 'microwavesinto two paths. The microwaves in the first or transmission path 39 passfrom yhorn 51'to horn 53. The microwaves in the second or reference path41 join with the microwaves from the first path to produce aninterference signal at detector 25 that has its 'principal Fouriercomponent at the harmonic of themodulation frequency nearest to N,

arising at the. output junction of the dispersive bridge as a result ofthe frequencyexcursion of the frequency modulated oscillator (Le. as aresult of the frequency modulation ofthe microwave oscillators 29 and 31together with thcextra wave guide 49 as observed when the sample 1 5 .isstatic and transparent to the microwaves). This produces aninterferencesignal having a phase modulation ofthis principal Fou'rier'component,and this phase modulation of this principal Fourier component isrecovered by feeding this interference signal to a tuned amplifierpassing only this one harmonic, and feeding this am- -plitier outputseriatim to a discriminator.integrator chain.

To this end the phase shift caused by plasma 15 in arm 23 produces aphase modulation of this component in accordancel with this inventionand thephase modulation is recovered by feeding the output signal fromthe detector 25 to tuned amplifier 69 which passes only this oneharmonic and transmitting this harmonic to a discriminator integratorchain wherein the low frequency response is set by the integration timeconstant. The high frequency response is limited by thecarrier-frequency aspect of the (fundamental)-modulation frequency. l

As will be understood fromA the above, this invention contemplates theinteraction with electromagnetic fields (microwaves) and the measurementof the free electron concentration as a functionof the frequency atwhich this measurement is made. No interference signals which aresinusoidal functions of phase shift are involved in the output. lNeitherare resonance frequencies, phasepropor tional display by means off araster system or unbalance feedbacks to a frequency modulated oscillatorinvolved.

In contrast to the heretofore known system, where it was impossible-toprovide a continuous output directly proportional to the plasma densitybelow the critical plasma density and unambiguous when the'density isabove critical, it' is possible with the system of this invention to dothis by operation of the described system with `a small Nv isapproximately -unity and the amplifier is tuned to the fundamental.Thus, the system of this invention has the advantage that it givesdirect voltage output and of simple operation at a high modulationfrequency.

What is claimed is:

1. A frequency modulated microwave system for meastiring the timevarying density of aplasma, comprislng producing frequency modulatedmicrowaves that are split in to a short. path and a long 4pathcontaining said plasma and joined to produce an interference signalcarrying a phase modulation correspondingto the phase-shift produced bypassing the microwaves through the plasma, said interferencesignalvhaving its principall Fourier cornponent' at theharrnonic N ofthemodulation frequency where N represents the number of interferencecycles arising when said split microwaves are joined as a result of thefrequency modulation excursion of the microwaves in passing through saidplasma, recovering said Fourier component and integrating it to producea continuous output signal directly corresponding to said plasma deni'sity when said plasma is transparent, while indicating unambiguouslywhen the plasma is opaque.

2. The system vof claim 1 wherein said modulation is equal to M, saidlong path is equal vto Lland ML approximatelycquals W vwhere; Wi'is thewavelength of said micro-waves that are modulated.

3'. The system of claim l wherein the microwaves have a frequency of35,000 megacycles persecond, the modula-v i tion thereof has-a frequencyof 30 rnegacycles per second, the time rate of change of the plasmadensity produces a change in frequencies within a band of frequencies of2 megacycles per second on either side of said modulation frequency, andsaid time integration is equal to one second.

References Cited by the Examiner UNITED STATES PATENTS 2,767,373 10/1956Maggio 324-67 2,798,197v 7/1957 Thurston S24- 58.5 2,971,153

2/,1961 Wharton et al 324-58.S

WALTER CARLSON, Primary Examiner.

A. E. RCHMOND, Assistant Examiner.

1. A FREQUENCY MODULATED MICROWAVE SYSTEM FOR MEASURING THE TIME VARYINGDENSITY OF A PLASMA, COMPRISING PRODUCING FREQUENCY MODULATED MICROWAVESTHAT ARE SPLIT INTO A SHORT PATH AND A LONG PATH CONTAINING SAID PLASMAAND JOINED TO PRODUCE AN INTERFERENCE SIGNAL CARRYING A PHASE MODULATIONCORRESPONDING TO THE PHASE-SHIFT PRODUCED BY PASSING THE MICROWAVESTHROUGH THE PLASMA, SAID INTERFERENCE SIGNAL HAVING ITS PRINCIPALFOURIER COMPONENT AT THE HARMONIC N OF THE MODULATION FREQUENCY WHERE NREPRESENTS THE NUMBER OF INTERFERENCE CYCLES ARISING WHEN SAID SPLITMICROWAVES ARE JOINED AS A RESULT OF THE FREQUENCY MODULATION EXCURSIONOF THE MICROWAVES IN PASSING THROUGH SAID PLASMA, RECOVERING SAIDFOURIER COMPONENT AND INTEGRATING IT TO PRODUCE A CONTINUOUS OUTPUTSIGNAL DIRECTLY CORRESPONDING TO SAID PLASMA DENSITY WHEN SAID PLASMA ISTRANSPARENT, WHILE INDICATING UNAMBIGUOUSLY WHEN THE PLASMA IS OPAQUE.